Direct current transformer employing magnetoresistance diodes for use in current measurement

ABSTRACT

This is a direct current transformer wherein there is inserted through the interior space of a C-shaped core a primary conductor through which there flows a direct current; there is disposed a magnetosensitive semiconductor device in a space defined between both ends of said core; a difference between the value of voltages obtained in proportion to the magnetic fluxes generated by the direct current travelling across both terminals of the magnetosensitive semiconductor device and the value of output voltages obtained from a constant voltage element serially connected to the magnetosensitive semiconductor device without being affected by the magnetic fluxes, is amplified by a differential amplifier; a current determined by said voltage difference is impressed, after being amplified, on a coil wound around part of the C-shaped core to generate such magnetic fluxes as offsetting the magnetic fluxes produced in the core by an electric current introduced through the primary conductor thereby always to maintain the magnetic fluxes prevailing in the core substantially at zero and current outputs from the differential amplifier are measured to find the value of the direct current passing through the primary conductor.

United States Patent Nakamura Mar. 14, 1972 [72] Inventor: TetsujiNakamura, Tokyo, Japan [73] Assignee: Tokyo Shibaura Electric Co., Ltd.,

Kawasaki-shi, Japan [22] Filed: Apr. 8, 1969 [21] Appl. No.: 814,260

3,519,899 7/1970 Yamada ..324/46 X 3,535,626 10/1970 Uemura et al...324/46 FOREIGN PATENTS OR APPLICATIONS 618,580 6/1957 Canada ..324/117Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Emest F. KarlsenAttorney-Flynn & Frishauf [5 7] ABSTRACT This is a direct currenttransformer wherein there is inserted through the interior space of aC-shaped core a primary conductor through which there flows a directcurrent; there is disposed a magnetosensitive semiconductor device in aspace defined between both ends of said core; a difference between thevalue of voltages obtained in proportion to the magnetic fluxesgenerated by the direct current travelling across both terminals of themagnetosensitive semiconductor device and the value of output voltagesobtained from a constant voltage element serially connected to themagnetosensitive semiconductor device without being affected by themagnetic fluxes, is amplified by a differential amplifier; a currentdetermined by said voltage difference is impressed, after beingamplified, on a coil wound around part of the C-shaped core to generatesuch magnetic fluxes as offsetting the magnetic fluxes produced in thecore by an electric current introduced through the primary conductorthereby always to maintain the magnetic fluxes prevailing in the coresubstantially at zero and current outputs from the differentialamplifier are measured to find the value of the direct current passingthrough the primary conductor.

10 Claims, 11 Drawing Figures PATENTEDHAR 14 1972 SHEET 2 BF 3 FIG. 6

FIG. 7

FIG.

PATENTEDHAR 14 I972 SHEET 3 OF 3 PEG. 8

DIRECT CURRENT TRANSFORMER EMPLOYING MAGNETORESISTANCE DIODES FOR USE INCURRENT MEASUREMENT BACKGROUND OF THE INVENTION The present inventionrelates to improvements in a direct current transformer using amagnetosensitive semiconductor device.

A direct current transformer heretofore used in measuring a large directcurrent at the primary side by converting it at the secondary side to asmall current adapted for measurement includes, for example, anapparatus using saturable reactors. In this apparatus, a pair ofsaturable cores penetrated by a primary bus are respectively wound witha secondary coil. The terminal ends of both coils are seriallyconnected. One end of an AC source is connected to the initial end ofone of the secondary coils and the other end of said source to theinitial end of the other secondary coil through an AC ammeter.

The aforesaid paired secondary coils generate electromotive forces whoseopposite polarities correspond to those of magnetic fluxes produced inthe paired saturable cores by a direct current running through theprimary bus, so that these magnetic fluxes offset each other.

In a direct current transformer involving saturable reactors bearing theaforementioned relationship with each other, one of the saturablereactors presents impedance to one of the half waves of exciting currentfrom the AC source and the other saturable reactor also displaysimpedance to the other half wave of said exciting current. Theseimpedances vary with the magnitude of a large direct current at theprimary side, namely, the larger the direct current, the more reducedwill be the impedance. Accordingly, the secondary alternating currentpassing through the secondary coils is made proportionate to themagnitude of the direct current, so that measurement of said secondaryalternating current by ammeter indicates the value of the originaldirect current.

Nevertheless, a direct current transformer using the aforementionedsaturable reactors has the drawbacks described below. To begin with, thedifierent compositions of the material of the paired cores often lead tovaried properties, so that there is required time in making correctcalibration using a special instrument. In the second place, thesaturable reactor is readily affected by the surrounding magnetic field,resulting in gross errors of measurement. Therefore, the requiredprevention of such errors restricts the shape and arrangement of adirect current bus. In the third place, there are also caused errors ofmeasurement by variations in the frequency, voltage, waveform or thelike of an AC source used in exciting the secondary coil, presentingdifficulties in accurate measurement of a direct current. In the fourthplace, there is a wide divergence of winding between the primary coil orbus and the secondary coil. If, therefore, a direct current passingthrough the bus contains a pulsating component as does the outputcurrent, for example, from a thyristor rectifier, then there will beinduced a great surge voltage into the secondary coil, leading to thedestruction of interphase insulation.

Another prior an direct current transformer involves a Hall element.According to this apparatus, the Hall element is inserted into theinterior space of the core. The magnetic fluxes generated in the core bya primary current are allowed to pass through the Hall element disposedin said space. When a direct current flows across the terminals providedon one pair of opposite sides there is obtained a Hall voltage at theterminals positioned on the other pair of opposite sides. Since the Hallvoltage is proportionate to the value of the primary current,measurement of said voltage indicates the value of the primary current.

However, an apparatus using the aforementioned Hall element has theundermentioned shortcomings. First, the Hall voltage is so weak that itmust be sufficiently amplified for use in measurement, requiring a highamplification unit. Such amplifier is subject to various inductiveactions, leading to errors of measurement. Secondary, there has beendevised an apparatus which involves a large number of Hall elementsscattered at various parts of the core with the view of increasing theHall voltage. However, such apparatus causes the construction of a coreto be complicated, presenting disadvantage in fabrication.

SUMMARY OF THE INVENTION The object of the present invention is toprovide a direct current transformer having the undermentionedadvantages by eliminating the aforementioned drawbacks encountered withthe conventional apparatus, wherein the fact is utilized that amagnetosensitive semiconductor device (hereinafter referred to as MSD")varies in the resistance presented in its forward direction inaccordance with the amount of magnetic flux travelling through saiddevice; a difference between the value of output voltages from MSDcorresponding to variations in the magnetic flux generated in the coreby a primary current and the value of output voltages obtainedindependently of said variations in the magnetic flux from a constantvoltage element serially connected to MSD is impressed, after beingamplified by a differential amplifier, on a coil wound around part ofthe C-shaped core, through the interior space of which is inserted aprimary conductor so as to cause the magnetic flux produced by thedirect current flowing through the primary conductor to be offset by themagnetic flux generated by a current passing through the coil.

The advantages offered by the direct current transformer of the presentinvention are:

1. The magnetic fluxes generated in the core by a direct currenttravelling through a direct current bus are offset by the magneticfluxes produced in said core by a feed back current flowing through asupplementary coil wound around said core so as to use the core with themagnetic fluxes present therein reduced almost to zero, thus making itpossible to minimize the cross sectional area of the core.

2. High sensitivity is obtained in measurement, because it is carriedout at around zero point of magnetic fluxes at which MSD displays aprominent magnetosensitivity.

3. A direct current can be measured with extremely high sensitivity oraccuracy by elevating the degree of amplification performed by adifferential or ordinary amplifier. Further, adjustment of the degree ofamplification enables the accuracy,

with which a direct current should be measured, to be varied freely asdesired.

4. Separation of the core into a main core member SR, and supplementarycore member SR eliminates the induction of voltage into a supplementarycoil resulting from sudden change in a direct current ID passing througha primary bus, thus pennitting easy insulation of said supplementarycoil and preventing an amplifier from being impressed with a surgevoltage.

BRIEF EXPLANATION OF THE DRAWINGS FIG. 1 is a schematic arrangement ofthe main part of a direct current transformer according to an embodimentof the present invention;

FIG. 2 is a circuit diagram illustrative of the other main part of theinvention;

FIGS. 3 and 4 represent the properties of said embodiment forillustration of its function;

FIG. 5 is a schematic arrangement of the main part of a direct currenttransformer according to another embodiment of the invention;

FIGS. 6, 7, 8, 9 and 11 are schematic arrangements of the main parts ofdirect current transformers according to still other embodiments of theinvention; and

FIG. 10 is a waveform diagram illustrative of the function of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 represents afirst embodiment of the present invention. Character B denotes a directcurrent bus through which there flows a direct current to be measuredand ID shows the direct current passing through said bus B in theindicated direction. There is provided a core SR so as to form amagnetic passage surrounding the bus B. Part of the core has a voidspace G into which is inserted, as shown, one of the MSDs used, namely,MSD,. Apart from MSD there is disposed another unit MSD at a place freefrom the effect of a magnetic field produced in the core SR. The core SRis wound with a supplementary coil CL, which is excited by outputs froma separate amplifier DFA (FIG. 2).

Now let it be assumed that there is generated in said coil magneticfluxes 42B in such a direction as offsetting the magnetic fluxes Dproduced in the core SR by a direct current ID travelling through theprimary bus. There are additionally provided a storage battery BT andadjusting resistor RH to form a closed circuit involving plus terminalof storage battery BT adjusting resistor RH terminal A of MSD terminal Kof MSD terminal A of MSD terminal K of MSD minus terminal of storagebattery ET. The terminal A of MSD is connected to a voltage outputterminal 1, the contact of the terminal K of MSD, with the terminal A ofMSD is connected to the terminal 2, and the terminal K of MSD isconnected to the terminal 3. Now let the output voltage between theterminals 1 and 2 be denoted as V, and the output voltage between theterminals 2 and 3 be designated as V FIG. 2 illustrates a circuit forreceiving output voltages from MSD and MSD of FIG. 1 through theterminals 1, 2 and 3 to obtain amplified outputs in proportion thepotential difference between V, and V and feeding back outputsrepresenting said difference to the supplementary coil CL wound aroundthe core SR. The same parts of FIG. 2 as those of FIG. I are denoted bythe same numerals. MSD and MSD are supplied with a suitable current imby the storage battery BT which flows in their forward direction. Theterminals 1, 2 and 3 for voltage outputs from MSD and MSD are connectedto the input terminals 6, 7 and 8 respectively of a separately provideddifferential amplifier DFA as 1 to 6, 2 to 7 and 3 to 8. One directcurrent output terminal 9 of the differential amplifier DFA is connectedto one terminal 4 of the supplementary coil CL through a DC ammeter DAand the other terminal 10 of said amplifier DFA to the other terminal 5of said ammeter DA. Now let it be as sumed that direct current outputsib from the differential amplifier DFA flows in the direction of9 DA 4CL 5 10, and that the differential amplifier DFA is actuated to anextent corresponding to a difference between the input voltage V, acrossits input terminals 6 and 7 and the input voltage V across its inputterminals 7 and 8, namely, direct current outputs rising with increasingvalue of AV lV V I.

There will now be described the operation of a direct currenttransfomier according to the aforementioned embodiment. MSD, and MSDused are of such type that where there is no magnetic field actingthereon, they display an equal resistance in the forward direction. Theresistances presented by MSD, and MSD in the forward direction aredenoted as RD, and RD: respectively, and the value of an adjustingresistor RH is set at a level fully higher than a sum of RD,+RD Now letit be assumed that slight variations in RD, and RD do not substantiallyeffect a current im supplied by the storage battery BT. If a directcurrent ID flows through the primary bus B and in consequence magneticfluxes d penetrate MSD, its magnetosensitivity will cause RD, to bereduced with the resultant decrease in the voltage V,. On the otherhand, MSD is positioned at a place free from the effect of the magneticfluxes D, so that the resistance RD in its forward direction does notchange (if necessary, MSD may be housed in an iron case so as to keep itfrom the effect of an external magnetic field). Accordingly, when thedirect current ID or the resultant magnetic fluxes D vary, the outputvoltage 1 across the voltage output terminals 1 and 2 decreases ininverse proportion to increases in the magnetic fluxes (bl), while thevoltage V across the voltage output terminals 2 and 3 indicates aconstant value, as shown in FIG. 3. Thus the voltage difference AV risesin proportion to increases in the magnetic fluxes D. The differentialamplifier DFA receives inputs V and V across the input terminals 6 and 7and input terminals 7 and 8 respectively, and direct current outputs ibfrom said am plifier DFA rise with increasing AV. The differentialamplifier DFA has such a property that in case of AV=O, ib will also bereduced to zero. The current outputs from said amplifier DF A have suchcharacteristics as shown by the dotted line of FIG. 3. When the currentoutputs ib from the differential amplifier DFA are introduced into thesupplementary coil CL there will be generated magnetic fluxes btravelling in an opposite direction to the magnetic fluxes D. If thenumber of windings of the supplementary coil CL and the volume ofcurrent outputs ib are suitably selected, the resultant magnetic fluxes418 will be able fully to offset the magnetic fluxes 41D. If thedifferential amplifier DFA has a fully high rate of amplification GA,then the current outputs ib will rise to the extent that the magneticfluxes B can substantially offset the magnetic fluxes qbD. Accordingly,there is established an equilibrium when the magnetic fluxes in thespace G of the core SR are reduced to zero as DB=0, and V and V are madesubstantially equal. If there is formed a feedback circuit for detectingvariations in the direct current ID flowing through the primary bus B,namely, the magnetic fluxes d D in the form of the voltage differencebetween MSD, and MSD and causing said magnetic fluxes (#0 to be offsetby the magnetic fluxes (#3 generated in the supplementary coil CL by theamplified current outputs from the differential amplifier DFA, thenthere will be obtained, as shown in FIG. 4, the characteristics ofcurrent outputs ib proportionate to the magnitude of the direct currentID travelling through the primary bus B. It will be apparent that thepresent invention is also applicable in an automatic direct currentcontrol device wherein the DC ammeter DA is replaced by a suitabledetector for measuring the magnitude of current outputs ib, and aseparately provided amplifler is supplied with signals proportionate tothe magnitude of current outputs 12 to control a source of the directcurrent ID by the resultant outputs from said amplifier, therebyautomatically to control the magnitude of the direct current ID. Theobject of elevating the accuracy of measuring a direct current ID can beeasily effected by carrying out determination in the vicinity of zeropoint of magnetic fluxes at which MSD displays high magnetosensitivityand increasing the sensitivity of the differential amplifier DFA.

FIG. 5 represents a second embodiment of the present invention wherethere is inserted MDS into the core SR. As apparent from the foregoingdescription, MSD, is only required to be positioned at a place free fromthe magnetic fluxes generated in the core SR through the primary bus B,so that as shown in FIG. 5, MSD may be set in a small window Wsurrounded by part of the core SR. The reason is that the magneticfluxes created in the core SR do not travel through the space withinsaid small window, but pass through that part of the core forming theperiphery of the window W, so that MSD is not affected by said magneticfluxes. There will be obtained a better effect if MSD, is inserted intothe small window in a suitable iron case. Connection of MSD, and MSD tothe storage battery BT and the feedback of a voltage difference betweenMSD, and MSD to the supplementary coil CL through the differentialamplifier DFA are carried out with the same circuit arrangement as inFIGS. 1 and 2.

As shown in FIG. 6, a third embodiment of the present invention, it ispossible to serially connect MSD, and a constant voltage diode ZD usedin place of MSD supply them with a current in: from the storage batteryBT. It is also possible to lead out the voltage output terminal 1 fromthe terminal A of MSD the voltage output terminal 2 from the contact ofthe terminal K of MSD, with the constant voltage diode ZD and thevoltage output terminal 3 from the contact of said diode ZD with theminus terminal of the storage battery ET for connection to thedifferential amplifier DFA as in FIG. 2. In this case, MSD is placed inthe space G defined between both ends of the core SR as in FIG. 1. Withthe voltage across the voltage output terminals 1 and 2 represented by Vand the voltage of the constant voltage diode across the voltage outputterminals 3 and 4 by V then it is possible to carry out exactly the sameoperation as in FIGS. I and 2 and measure the direct current ID by theoutput ib from the differential amplifier DFA, which corresponds to AV=VV There will now be described a fourth embodiment of the presentinvention by reference to FIG. 7. In this case comparison is madebetween the voltage of MSD, and the referential voltage of a separatelyprovided power source and the resultant voltage difference is amplified.MSD disposed in the space defined between both ends of the core SR issupplied with a suitable current im from the storage battery BT. Asshown in FIG. 7, the terminal A of MSD is connected to one terminal ofthe constant voltage diode ZD. A terminal is led out from the otherterminal of said diode ZD, and a terminal from the terminal K of MSD,.Both terminals of the constant voltage diode ZD are supplied through aresistor R with a direct current from a separately provided directcurrent source DC as shown (said source supplies through a resistor R adirect current to the constant voltage diode ZD by way of a route: analternating current source AC a supplementary transformer Tr a rectifierRF for supplying a direct current) so as to impress a constant voltage Von said diode ZD. With the terminal voltage of MSD, denoted as V,, thenthere appears across 10 and 20 a voltage difierence AV=V -V Said voltagedifference is amplified by an amplifier AMP and the terminals associatedwith resultant direct current output are connected to the terminals 4and 5 respectively of the supplementary coil CL of FIG. 1. Sucharrangement enables a feedback circuit supplying a direct current fromthe amplifier AMP to the supplementary coil to be controlled to AV O,consequently DB=0 just as in the case of FIGS. 1 and 2, and themagnitude of the direct current ID to be determined from the output ibof the amplifier AMP. Since, in this case, the current im supplied toMSD, should be kept as constant as possible, it is prepared that thestorage battery BT be replaced by a suitable constant current sourceapparatus.

There will now be described by reference to FIG. 8 a fifth embodiment ofthe present invention where a plurality of (for example, four asillustrated) MSDs are employed. Through the window of the core SR isinserted a direct current bus, through which there flows a directcurrent ID as in FIG. 1. The core SR has two spaces G and G formed asshown. In these spaces G 1 and G are disposed MSD, and MSD respectively,as shown, in a manner to assume the same polarity as that of themagnetic fluxes D generated in the core SR by the direct current ID. Asin FIG. 1, the supplementary coil CL is wound around the core SR andexcited by outputs from an amplifier or the like to produce magneticfluxes B of opposite polarity to the magnetic fluxes D generated in thecore itself. There is formed a closed circuit extending from the plusterminal to the minus terminal of the storage battery ET in which acurrent travels through adjusting resistor RH terminal A of MSD terminalK of MSD, terminal A of MSD terminal K of MSD and then through theterminals A and K of MSD and MSD. respectively which are disposed at apoint free from the effect of magnetic fluxes generated in the core SR,so as to introduce a suitable current im. There is led out a voltageoutput terminal I from the terminal A of MSD a voltage output terminal 2from the contact of the terminal K of MSD, with the terminal A of MSDand a voltage output terminal 3 from the terminal K of MSD,,. With thevoltage across said voltage output terminals 1 and 2 represented by V,,and the voltage across said voltage output terminals 2 and 3 by V thevoltage V, varies with the direct current ID, while the voltage Vremains constant independently of the magnitude of said direct currentID. As in FIG. 2, the voltage output terminals 1, 2 and 3 are connectedto the input terminals 6, 7 and 8 respectively of the differentialamplifier DFA shown in this figure and the output terminals of saidamplifier DFA to the terminals 3 and 5 respectively of the supplementarycoil CL. As in FIGS. I and 2, such arrangement enables the supplementarycoil CL to be excited by an exciting current ib corresponding to AV=V Vin proportion to the direct current ID, and the magnitude of said directcurrent to be determined from the readings of the output current ib.

The foregoing embodiment relates to the case where the core SR has twospaces in which there are positioned two MSDs. However, if there areformed more spaces in the core SR and MSDs are increased to thecorresponding number, there will be obtained exactly the same effect.Use of serially connected MSDs in such large numbers will enable theoutput voltage V which varies with changes in the magnetic fluxesproduced in the core SR, to be much more increased than when a singleMSD is employed. Accordingly, it is possible to obtain ahigh-sensitivity apparatus which can prominently display the value of AV=V,V even at slight variations in the magnetic fluxes 45D. Further,unifonn arrangement of spaces in the core SR will all the more reducethe effect of a sur rounding magnetic field, thus improving the accuracyof measuring the magnitude of the direct current ID.

FIG. 9 represents a sixth embodiment of the present invention, namely,an MSD'type direct current measuring apparatus using a supplementarysource of alternating current. As illustrated, this embodiment involvestwo MSDs connected in inverse parallel, that is, the terminal A of MSDis connected to the terminal K of MSD and the terminal K of MSD, to theterminal A of MSD In addition to these MSDs, there are disposed two moreunits, namely, MSD; and MSD. at a place free from the effect of themagnetic fluxes generated in the core by the direct current ID. Thelatter two MSDs form an inverse parallel circuit by connecting theterminal A of MSD to the terminal K of MSD, and the terminal K of MSD tothe terminal A of MSD Thus there is formed a closed circuit extendingfrom one terminal to the other of the supplementary alternating currentsource AC in which a current flows through adjusting resistor RI-Iterminal A of MSD terminal K of MSD terminal A of MSD: terminal K of MSDThere is led out a voltage output terminal I from the terminal A ofMSD,, a voltage output terminal 2 from the terminal K of MSD and avoltage output terminal 3 from the terminal K of MSD The voltage acrosssaid voltage output terminals l and 2 is designated as V, and thevoltage across the voltage output terminals 2 and 3 as V When MSD, andMSD are affected by the magnetic fluxes D generated in the core SR bythe direct current ID, their resistance in the forward direction willfall to cause a slight voltage drop to be presented with respect to acurrent im supplied by an alternating current source AC, with the resultthat there occurs a voltage difference as shown by the hatching of FIG.10 between the voltage 1 across MSD and MSD and the voltage 2 across MSDand MSD which are located free from the effect of the magnetic fluxes D.Said voltage difference is supplied to the input terminals of adifferential amplifier like that of FIG. 2 to produce outputs in theform of a direct current and said current is supplied to the terminals 4and S of the supplementary coil CL wound around the core SR of FIG. 1.Thus the direct current ID can be determined in exactly the same manneras in FIGS. 1 and 2.

There will now be described by reference to FIG. II a seventh embodimentof the present invention where the direct current transformer comprisesa main core member and a supplementary core member combined with MSDs.As illustrated, there is disposed the main core member SR in a manner tosurround a direct current bus B through which there travels a directcurrent ID. In the space defined between both ends of said main core SRis positioned one MSD, namely, MSD A separately provided supplementarycore SR is wound with a supplementary coil CL, and in the space definedbetween both ends of said supplementary core SR is disposed another MSD,or MSD There is formed a closed circuit extending from the plus terminalto the minus terminal of a supplementary direct current source ET inwhich a current im travels through adjusting resistor RH terminal A ofMSD terminal K of MSD terminal A of MSD terminal K of MSD There is ledout a voltage output terminal 1 from the terminal A of MSD a voltageoutput terminal 2 from the contact of the terminal K of MSD with theterminal A of MSD and a voltage output terminal 3 from the terminal K ofMSD The voltage across said voltage output terminals 1 and 2 is denotedas V and the voltage across said voltage output terminals 2 and 3 as VThese voltage output terminals 1, 2 and 3 are connected to the inputterminals 6, 7 and 8 respectively of the differential amplifier DFA as lto 6, 2 to '7 and 3 to 8. The output terminal 9 of the differentialamplifier DFA is connected through a DC ammeter DA to the terminal 4 ofthe supplementary coil CL and the terminal 10 of said amplifier DFA tothe terminal 5 of said coil CL. The magnetic fluxes D provided in themain core member SR by the direct current [D and the magnetic fluxes qSBgenerated in the supplementary core SR through the supplementary coil CLby the outputs ib from the differential amplifier DFA are of suchpolarity as exerting a polar action on MSD and MSD in the same directionas their respective polarities. Now let it be assumed that thedifferential amplifier DFA produces current outputs ib in such a manneras to balance the input voltage V, across the input terminals 6 and 7with the input voltage across the input terminals 7 and 8. If, in thisembodiment, a direct current lD flowing through the bus B causes V todecrease according to variations in the resistance RD, of MSD, in itsforward direction, then there will occur an unbalance between the inputvoltages V, and V of the differential amplifier DFA, increasing thecurrent outputs ib accordingly. Thus the magnetic fluxes generated inthe supplementary core SR increase to cause the resistance RD of MSD inits forward direction to vary in the same manner as the resistance RD ofMSD with the resultant drop of V and settle at a point of V V Since thecurrent outputs ib from the differential amplifier DFA are so varied asto always maintain the condition of V V there results the relationshipof ib 00 ID. Accordingly, the magnitude of the direct current lDtravelling through the bus B can be determined by reading ib from the DCammeter DA.

What is claimed is:

l. A direct current transformer comprising a C-shaped core excited by adirect current to be measured which flows through a conductorpenetrating the interior space of said core; a supplementary coil woundon part of the core for generating magnetic fluxes in such a directionas to offset the magnetic fluxes produced by the direct current; amagnetosensitive semiconductor device having rectifier characteristicsdisposed in the space defined between both ends of the C-shaped core; aconstant voltage element serially connected to the magnetosensitivesemiconductor device for producing a constant voltage output withoutbeing affected by the magnetic field prevailing in the core; a powersource for impressing a voltage on a circuit comprising themagnetosensitive semiconductor device and constant voltage elementconnected in series; a means for impressing the supplementary coil witha voltage which is a function of the difference between the voltageacross the magnetosensitive semiconductor device and the voltage acrossthe constant voltage element, said means for impressing providing anoutput current which is a function of the direct current to be measuredwhich current may be applied to a current-measuring device for providinga readout.

2. The direct current transformer according to claim 1 wherein theconstant voltage element is disposed within an opening in the core toobstruct the passage of magnetic fluxes.

3. The direct current transformer according to claim 1 wherein theconstant voltage element comprises a constant voltage diode impressedwith a constant voltage.

4. A direct current transformer comprising a closed circuit involving acore through the interior space of which is inserted a conductorreceiving a direct current to be measured and having a plurality ofseparate spaces formed in part of said core; a coil wound around part ofthe core for generating magnetic fluxes in such a direction as to offsetthe magnetic fluxes produced by the direct current; a plurality ofmagnetosensitive semiconductor devices having rectifier characteristicspositioned in the separate spaces and connected in such a manner thateach of them has a polarity directed in the same way as the magneticfluxes prevailing in the core; mutually serially connected constantvoltage elements having the same number as said plurality ofmagnetosensitive semiconductor devices, serially connected to the outputterminals of said magnetosensitive semiconductor devices and so disposedas to be kept free from the effect of the magnetic field prevailing inthe core; a power source for impressing a voltage on a circuitcomprising said plurality of magnetosensitive semiconductor devicesmutually connected in series and another circuit comprising saidplurality of constant voltage elements mutually connected in series; anda means for impressing the supplementary coil with a voltage which is afunction of the difference between the voltage output from the outputterminals of the circuit comprising said plurality of magnetosensitivesemiconductor devices and the voltage output from the output terminalsof the circuit comprising said plurality of constant voltage elements,said means for impressing providing an output current which is afunction of the direct current to be measured which current may beapplied to a current-measuring device for providing a readout.

5. A direct current transformer comprising a first C-shaped core excitedby a direct current to be measured which flows through a conductorpenetrating the interior space of said core; a second C-shaped corelocated at a point free from the effect of the magnetic fluxesprevailing in said first C-shaped core; a first magnetosensitivesemiconductor device having rectifier characteristics disposed in thespace defined between both ends of said first C-shaped core and varyingin its forward resistance in accordance with the change at said space ofthe magnetic fluxes generated in the core by the direct current flowingthrough the conductor; a second magnetosensitive semiconductor devicehaving rectifier characteristics posi tioned in the space definedbetween both ends of said second C-shaped core and serially connected tosaid first magnetosensitive semiconductor device; a supplementary DCpower source for supplying a current to a circuit comprised of saidserially connected first and second magnetosensitive semiconductordevices; a coil wound around the second C-shaped core for generatingmagnetic fluxes in such a manner as to cause them to exert a polaraction on said first and second magnetosensitive semiconductor devicesin the same direction as their respective polarities; and a meansincluding a variable amplification factor differential amplifier forimpressing the coil with the voltage difference between the voltageoutputs from the first magnetosensitive semiconductor device and thevoltage outputs from the second magnetosensitive semiconductor device insuch a manner that the voltages prevailing across both ends of therespective first and second magnetosensitive semiconductor devices aremade equal by said voltage difference, whereby the value of a currentintroduced through a DC ammeter due to said voltage difference ismeasured to find the value of the direct current traveling through theconductor.

6. The direct current transformer according to claim 1 wherein theforward resistance of said magnetosensitive semiconductor varies inresponse to said magnetic field.

7. The direct current transformer according to claim 4 wherein theforward resistance of said magnetosensitive semiconductor varies inresponse to said magnetic filed.

8. The direct current transformer according to claim 5 wherein theforward resistances of said magnetosensitive semiconductors vary inresponse to said magnetic field.

9. The direct current transformer according to claim I wherein saidmeans for impressing said supplementary coil with said voltagedifference includes a differential amplifier having a variableamplification factor receiving said constant having a variableamplification factor receiving said constant voltages and the outputvoltages from said magnetosensitive devices, the accuracy of saidtransformer being varied by varying said amplification factor.

1. A direct current transformer comprising a C-shaped core excited by adirect current to be measured which flows through a conductorpenetrating the interior space of said core; a supplementary coil woundon part of the core for generating magnetic fluxes in such a directionas to offset the magnetic fluxes produced by the direct current; amagnetosensitive semiconductor device having rectifier characteristicsdisposed in the space defined between both ends of the C-shaped core; aconstant voltage element serially connected to the magnetosensitivesemiconductor device for producing a constant voltage output withoutbeing affected by the magnetic field prevailing in the core; a powersource for impressing a voltage on a circuit comprising themagnetosensitive semiconductor device and constant voltage elementconnected in series; a means for impressing the supplementary coil witha voltage which is a function of the difference between the voltageacross the magnetosensitive semiconductor device and the voltage acrossthe constant voltage element, said means for impressing providing anoutput current which is a function of the direct current to be measuredwhich current may be applied to a current-measuring device for providinga readout.
 2. The direct current transformer according to claim 1wherein the constant voltage element is disposed within an opening inthe core to obstruct the passage of magnetic fluxes.
 3. The directcurrent transformer according to claim 1 wherein the constant voltageelement comprises a constant voltage diode impressed with a constantvoltage.
 4. A direct current transformer comprising a closed circuitinvolving a core through the interior space of which is inserted aconductor receiving a direct current to be measured and having aplurality of separate spaces formed in part of said core; a coil woundaround part of the core for generating magnetic fluxes in such adirection as to offset the magnetic fluxes produced by the directcurrent; a plurality of magnetosensitive semiconductor devices havingrectifier characteristics positioned in the separate spaces andconnected in such a manner that each of them has a polarity directed inthe same way as the magnetic fluxes prevailing in the core; mutuallyserially connected constant voltage elements having the same number assaid plurality of magnetosensitive semiconductor devices, seriallyconnected to the output terminals of said magnetosensitive semiconductordevices and so disposed as to be kept free from the effect of themagnetic field prevailing in the core; a power source for impressing avoltage on a circuit comprising said plurality of magnetosensitivesemiconductor devices mutually connected in series and another circuitcomprising said plurality of constant voltage elements mutuallyconnected in series; and a means for impressing the supplementary coilwith a voltage which is a function of the difference between the voltageoutput from the output terminals of the circuit comprising saidplurality of magnetosensitive semiconductor devices and the voltageoutput from the output terminals of the circuit comprising saidplurality of constant voltage elements, said means for impressingproviding an output current which is a function of the direct current tobe measured which current may be applied to a current-measuring devicefor providing a readout.
 5. A direct current transformer comprising afirst C-shaped core excited by a direct current to be measured whichflows through a conductor penetrating the interior space of said core; asecond C-shaped core located at a point free from the effect of themagnetic fluxes prevailing in said first C-shaped core; a firstmagnetosensitive semiconductor device having rectifier characteristicsdisposed in the space defined between both ends of said first C-shapedcore and varying in its forward resistance in accordAnce with the changeat said space of the magnetic fluxes generated in the core by the directcurrent flowing through the conductor; a second magnetosensitivesemiconductor device having rectifier characteristics positioned in thespace defined between both ends of said second C-shaped core andserially connected to said first magnetosensitive semiconductor device;a supplementary DC power source for supplying a current to a circuitcomprised of said serially connected first and second magnetosensitivesemiconductor devices; a coil wound around the second C-shaped core forgenerating magnetic fluxes in such a manner as to cause them to exert apolar action on said first and second magnetosensitive semiconductordevices in the same direction as their respective polarities; and ameans including a variable amplification factor differential amplifierfor impressing the coil with the voltage difference between the voltageoutputs from the first magnetosensitive semiconductor device and thevoltage outputs from the second magnetosensitive semiconductor device insuch a manner that the voltages prevailing across both ends of therespective first and second magnetosensitive semiconductor devices aremade equal by said voltage difference, whereby the value of a currentintroduced through a DC ammeter due to said voltage difference ismeasured to find the value of the direct current traveling through theconductor.
 6. The direct current transformer according to claim 1wherein the forward resistance of said magnetosensitive semiconductorvaries in response to said magnetic field.
 7. The direct currenttransformer according to claim 4 wherein the forward resistance of saidmagnetosensitive semiconductor varies in response to said magneticfield.
 8. The direct current transformer according to claim 5 whereinthe forward resistances of said magnetosensitive semiconductors vary inresponse to said magnetic field.
 9. The direct current transformeraccording to claim 1 wherein said means for impressing saidsupplementary coil with said voltage difference includes a differentialamplifier having a variable amplification factor receiving said constantvoltage and the output voltage from said magnetosensitive device, theaccuracy of said transformer being varied by varying said amplificationfactor.
 10. The direct current transformer according to claim 4 whereinsaid means for impressing said supplementary coil with said voltagedifference includes a differential amplifier having a variableamplification factor receiving said constant voltages and the outputvoltages from said magnetosensitive devices, the accuracy of saidtransformer being varied by varying said amplification factor.